1. Field of the Invention
The present invention relates to an external storage system used in computer systems, and in particular to an external storage system which has a plurality of storage devices and a storage control device for controlling those storage devices and which is suitable for hierarchically managing storage space of a plurality of storage devices without intervention of a host apparatus (or initiator device) and thereby realizing a large capacity and a low bit cost without casting a burden upon the host apparatus.
2. Description of the Related Art
Conventionally, techniques for moving and storing respective data in suitable storage locations according to use frequency of respective data have widely been studied and developed as techniques of data management for making efficient use of limited storage space. For example, in JP-A-60-147855 and GH35-0092-5, "Data Facility Hierarchical Storage Manager Version 2 Release 5.0: General Information" published by IBM in 1989, automatic data management techniques in computer systems each having a plurality of external storage devices such as high-performance high-cost storage devices and low-performance low-cost storage devices connected to a host apparatus are described.
If the above described conventional techniques are used and there is no access of the host apparatus to data stored in a high-performance high-cost storage device for a predetermined time, this data is compressed by software stored in the host apparatus and thereafter transferred to a low-performance low-cost storage device and stored therein. Further, if there is access of the host apparatus to a low-performance low-cost storage device, this data is expanded and thereafter transferred to a high-performance high-cost storage device.
Every processing for data management as heretofore described is performed by "hierarchical storage management program" which is continuously executed in the host apparatus. Various external storage devices of a comparatively large-scale system are automatically managed, and optimization of use of the data storage space is aimed at.